This invention relates to a process for the quaternization of membranes after formation from polymers and copolymers having active groups subject to quaternization. More particularly, it relates to a process for quaternizing hollow fiber membranes after formation from such polymers and copolymers.
Anion exchange membranes have utility in many applications involving diffusion, dialysis and electrodialysis of fluids. Among the uses for which anion exchange membranes are suitable is the separation of anions from solutions containing cations and competing anions, i.e. industrial waters and waste waters such as mine wastes, plating baths and cooling waters. Frequently such separation of anions utilizes the Donnan principle by which a solution of a multivalent ion contacting an ion exchange membrane will transfer the multivalent ion to a more concentrated solution of ions of a lower valency. Anion exchange membranes contain anion exchange groups either introduced into the membrane or developed therein by suitable treatment. In order to provide for exchange of anions, such membranes require the presence of cationic groups such as quaternary ammonium groups. Development of quaternary ammonium groups will depend on the nature of the active groups in the polymers comprising the membrane. If the polymer contains tertiary amino groups these can be reacted with alkyl halides to produce the desired quaternary ammonium groups; whereas if the polymer contains active haloalkyl groups they can be treated with tertiary amines to produce quaternary ammonium groups.
In the prior practice it has been known to prepare polymers and copolymers in resin form containing potentially active anion exchange sites and to react the polymers in such form to produce the anion exchange groups. It has been taught that such quaternization reactions as those with alkyl halides and tertiary amines can also be carried out after the formation of membranes from the potentially ionically active polymers. When such reactions have been conducted for sufficient periods to substantially quaternize the available reactive groups, the permeability and separation properties as well as the homogeniety of the membrane quaternized after formation are frequently improved over those properties of membranes formed from previously quaternized polymers. Consequently, quaternization after membrane formation is preferred.
In the instance of membranes produced in the form of hollow fibers, it has been found to be most desirable to convert the potentially ionically active sites to ionic form after formation of the hollow fiber membranes. Attempts to closely control the morphology and porosity of hollow fiber membranes if spun from solutions of ionically active polymers have not been notably successful. It has been proposed in U.S. Pat. No. 3,945,927 of Imai et al. to coat a porous substrate with a thin film of a potentially ionically active polymer and thereafter introduce ion-exchange groups to only the thin coating film to produce ion exchange composite membranes. Such a composite membrane utilizes only the thin coating film for anion exchange dialysis.
It has also been proposed in U.S. Pat. No. 3,944,485 of Rembaum et al. to impregnate a pre-formed hollow fiber with mixtures of polymerizable monomers and to polymerize such monomers to form cross-linked, insoluble ion-exchange resin particles embedded within the pores of the fiber. It has been further proposed to additionally aminate and quaternize such embedded, insoluble ion-exchange resin particles by Rembaum in U.S. Pat. No. 4,014,798. Previously known methods of amination and quaternization are suggested therein.
Various methods have been proposed in the past for carrying out such quaternization reactions. These include exposure to vaporized low molecular weight quaternizing agents as well as soaking in solutions of various concentrations of the quaternizing agents. The solutions have generally utilized solvents which do not take part or interfere with the quaternization such as water, alkanols, and hydrocarbons such as hexane, hexene, cyclohexane, benzene and the like. However, the more dense and compact the structure of the membranes cast or extruded the slower the quaternization reaction proceeds and the longer the time required to obtain substantial reaction of the active groups present in the polymer membranes. A convenient method of carrying out quaternization reactions on such previously formed hollow fiber membranes whereby substantial degrees of quaternization are readily obtained and the hollow fiber membranes display high ion exchange capacity is desired.